This invention relates to methods of reducing NOx, Sox, CO, CO2 and/or Hg emissions from combustion sites, and more particularly relates to the reduction of such emissions when a coal product or a hydrocarbon is being burned.
There is presently and will undoubtedly remain for years a need for the combustion of coal products and hydrocarbons to provide the energy desired throughout the world. Coal and hydrocarbons have been primary energy sources for decades and will no doubt remain even though alternative xe2x80x9ccleanxe2x80x9d energy sources are slowly being developed. In recent years, the effects upon the atmosphere from the discharge of NOx, SOx, CO and even CO2 have been intensely studied, and as a result, ever more stringent limits are being placed upon the discharge into the atmosphere of some or all of these products of combustion. The potential toxicity of mercury (Hg) has been known for some time, but only recently have there been imposed prohibitions against even minute discharges, which may be particularly troublesome when burning coal from certain deposits.
Heretofore, it has been possible to minimize the product ion of NOx, by minimizing the amount of air that is used, which of course supplies the N2 that is oxidized, and careful control has in some instances permitted the use of minimal or no excess air. However, in coal-fired boilers and the like where air is used to convey the coal to the combustion zone, in order to efficiently use that air in the combustion process, the parallel injection of fuel gas or the like is often required. This alternative of course raises costs by the differential between the cost of fuel gas and the cost of coal, per their BTU value. However, regardless of how efficiently such coal-fired boilers can be operated, such boilers and furnaces will continue to produce sulfur dioxide (SO2), nitrogen oxides (NOx,), carbon monoxide (CO) and carbon dioxide (CO2) emissions along with potential minute emissions of Hg, all of which generates photochemical smog and acid rain. Moreover, it is presently felt that such emissions constitute a cause of global warming which may have a destructive effect on vegetation and also cause the deterioration of the ozone layer, as a result of which humans become exposed to UV, resulting in lung damage and asthma.
The internal combustion engine used in cars, trucks, buses and the like throughout the world is another significant generator of NOx, and although catalytic converters are effective to reduce certain emissions, there is still significant amounts of NOx exiting from the exhaust pipes of such vehicles. U.S. Pat. No. 6,110,435 describes melting urea and then adding the molten product to exhaust gas to reduce NOx. This overall arrangement, however, leaves much to be desired, and superior solutions to the problem continue to be sought.
Overall, it can be foreseen in the coming decades that governmental agencies will continue to set stiffer standards from an environmental standpoint as to emissions being discharged into the atmosphere, and accordingly the search continues for improvements in reducing such emissions.
The invention provides a method for reducing NOx, SOx, Hg, CO2 and/or CO emissions from the combustion of coal products or hydrocarbons, when such coal products or hydrocarbons are burned and create hot combustion products, by contacting those products with particulate elemental iron at a temperature of at least about 300xc2x0 F., and preferably at least about 700xc2x0 F. when coal is being burned, so as to cause chemical and possibly physical reactions, as a result of which solid iron oxide and/or iron carbide and/or iron carbonate reaction products are produced that can be removed from the emission stream from such combustion. As a result of such reactions, the amounts of such undesirable gaseous emissions are substantially reduced or essentially eliminated.
In one particular aspect, the invention provides a method of reducing NOx, and/or CO emissions from the flue gas stream exiting from a coal-fired boiler, which method comprises burning a coal product to create hot gaseous combustion products, and exposing said gaseous combustion products to contact with particulate elemental iron at a temperature of at least about 700xc2x0 F. to cause said elemental iron to chemically react therewith, whereby such chemical reactions produce solid iron oxide and/or iron carbides and/or iron carbonates that are removed from the flue gas, which becomes reduced in NOx, and/or CO/CO2 content as a result of such chemical reactions.